The use of a turbocharger in a vehicle engine increases the power output of the engine for a given engine size. Turbochargers therefore reduce fuel consumption for a particular desired power output by engine downsizing and are therefore an important technology in reducing emissions of gases contributing to climate change and environmental pollution. When turbocharged smaller engines are used to replace bigger engines, they provide a similar performance with greater fuel economy. There has therefore been an upsurge in interest in the use of turbochargers.
The usefulness of conventional turbocharger technology comprising a centrifugal compressor is, however, limited by the operating range of the centrifugal compressor. The operating range is determined by the operating points at which the compressor experiences surge and choke, with the operating range being between these two points of operation. The phenomenon of “surge” is characterised by a reversal in fluid flow through the compressor, and occurs when the compressor is unable to force fluid against the pressure gradient on either side of its blades and to continue compressing air. It typically occurs as the pressure ratio in the compressor increases. The phenomenon of “choke” is characterised by a maximum in flow rate through the compressor. It typically occurs as the pressure ratio in the compressor decreases.
The operating range of the compressor in a turbocharger can be a limiting factor on the performance of a turbocharged engine. The limited operating range of the compressor means that the turbocharger will not perform well across a wide range of operating conditions. One solution is to use two or more turbochargers in a vehicle, one or more of them being optimised for high load and low r.p.m. of the engine, and at least one other for low load and high r.p.m. This solution has the drawbacks, however, of adding extra cost, weight and complexity to a vehicle containing these additional turbochargers. The control of two or more turbochargers is also more complex than controlling a single turbocharger. Attempts have therefore been made to control the flow of air into the compressor to decrease the mass flow rate at which surge occurs and to increase the mass flow rate at which choke occurs.
Casing treatment is one such flow control method. A channel is introduced in the compressor casing to encourage recirculation of high-pressure fluid at the impeller inlet. Swirl vanes may also be introduced within this recirculation channel. This casing treatment can decrease the mass flow rate at which surge begins, but it increases the complexity and cost of a turbocharger and is ineffective at low speeds.
A second method of flow control intended to, in effect, increase the operating range of a turbocharger, by shifting that operating range during use, is the use of variable inlet guide vanes. The angle of these vanes determine the angle of air flow at the compressor's impeller inlet. The vane angle is adjusted based on one or more operating condition. A drawback of this approach is this requirement for active control of the vane angle. If the vane angle is not adjusted according to operating conditions, they can have a throttling effect on the compressor. When the vanes are set at an angle which decreases the maximum mass flow value at which compressor surge occurs, they have a throttle effect which reduces the minimum mass flow value at which choke occurs. The operating range of the turbocharger is therefore not extended by the use of variable inlet guide vanes, but merely shifted.
It is therefore desirable to address these disadvantages.